Effect of Pleistocene Depositional Heterogeneity on Movement of a Subsurface Crude-Oil Spill, San Patricio County, South Texas

By

DUTTON, ALAN R., and HOVORKA, SUSAN D.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

BENNETT, PHILIP C.

Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Crude oil discharging into a coastal creek at a South Texas abandoned oil field was tracked through Pleistocene (Lissie Formation) fluvial channel deposits. Source of the subsurface oil spill is thought to have been an uncapped gathering line. The oil plume was mapped by using a soil probe/soil-vapor survey and analyses of UV fluorescence and TPH in 14 cores. Pleistocene deposits include northeast-oriented channel sandstone bodies (150 × 10 ft), clayey abandoned channel fill, and overbank deposits. Associated floodplain facies are characterized by clay beds and thin crevasse splay sands. A fairly continuous, 1- to 7-ft-thick silty clay bed records channel abandonment and separates an upper from two lower stacked channel sands.

The oil seep discharges from crevasse splay deposits oblique and adjacent to the channel sands; most of the oil lies in the lower channel-sand complex. The abandoned channel clays trap volatile petroleum-contaminant vapors, making the plume easy to map by soil probe/soil-vapor survey. Distribution of oil and reduction-discolored sediments, as well as gas chromatogram patterns, suggests that fluctuation in the water table smears oil between stratigraphic units and within the channel sand. A map of C1-C8 hydrocarbon vapors shows that oil is more degraded along the flanks than on the axis of the plume. Partly oxidized oil in upper channel-sand and floodplain deposits records the position of the plume during an intermittently high water table. Strongly reduced colors above and lateral to the oil plume probably reflect the reduced oxidation potential of ground water that contacted the oil.